1. Field of the Invention
The present invention relates to an image forming apparatus such as a copying apparatus, a printer or a facsimile apparatus, and more particularly to an image forming apparatus in which an image on an image bearing member is transferred onto a transfer material borne on a transfer material bearing member.
2. Related Background Art
With the recent popularization of the electrophotographic color image forming apparatus, there is an increasing requirement for the higher color output, in addition to the requirement for the higher recording quality of the color image. In order to meet such requirements, there are being proposed certain image forming methods, including a tandem color image forming method in which the images are formed by BK (black) toner, C (cyan) toner, M (magenta) toner and Y (yellow) toner respectively on drum-shaped image bearing members, then are transferred in succession by respective transfer means onto a transfer material conveyed by a transfer conveying belt and are finally fixed as disclosed for example in the Japanese Patent Application Laid-Open No. 9-288396.
In such tandem color image forming apparatus, as shown in FIG. 6, transfer materials set in a cassette 50 mounted in the lower right side portion of the main body of the apparatus are taken out, one by one, by a feed roller 51 and fed by paired rollers 52, 53 to an image forming unit.
In the image forming unit, a transfer conveying belt 54 for conveying the transfer material is stretched around plural rotary rollers to be formed in a planiform shape in the conveying direction of the transfer material (from right to left in FIG. 6), and, at the most upstream position, the transfer material is electrostatically attracted to the transfer conveying belt 54 by an attraction roller 55 to which a bias voltage is applied. Opposed to the conveying surface of the belt, there are linearly provided four drum-shaped image bearing members (hereinafter called photosensitive drums) 56a, 56b, 56c, 56d constituting image forming means. On each of the photosensitive drums 56a, 56b, 56c, 56d a charger and a developing device are provided along the periphery of the photosensitive drum, and a transfer member is disposed with interposing the conveying surface of the transfer conveying belt 54 between the photosensitive drum and the transfer member.
The casings of the above-mentioned developing devices respectively contain toner of yellow, magenta, cyan and black colors from the upstream side (right side in FIG. 6) in the conveying direction of the transfer material, and a color image is formed by transfer of the images of respective colors in succession to the transfer material conveyed by the transfer conveying belt 54. The transfer material is then conveyed to fixing means 57, positioned at the left-hand side of the apparatus, for image fixation and is discharged from the apparatus.
Such a color image forming apparatus does not necessarily always execute multi-color or full-color recording but the monochromatic recording with black color only is also executed considerably frequently. In order to accommodate such a mode of use, there is proposed a tandem color image forming apparatus capable of switching full-color recording and monochromatic recording.
For example, as shown in FIG. 7A, the transfer portions other than that for the black color are rendered retractable, together with the transfer conveying belt 54, by rotation about a support roller 58 which is positioned at the upstream side of the black photosensitive drum 56d and is contact with the internal surface of the transfer conveying belt 54. In case of monochromatic recording, the transfer portions other than that for the black color are retracted, together with the transfer conveying belt 54, by rotation as shown in FIG. 7B, whereby the black image alone is recorded. With such a configuration, the conveying path for the transfer material from the attraction roller 55 through the black image forming unit to the fixing means 57 remains same in the full-color recording and in the monochromatic recording, whereby stable conveying performance can be attained.
However, at the monochromatic recording with black color, the photosensitive drums other than that 56d for the black color are separated from the transfer conveying belt 54, so that the transfer material is conveyed without regulation over a longer distance before reaching the black toner image transfer portion. With such longer conveying distance, the charge of the transfer material supplied by the attraction roller 55 is gradually dissipated from the transfer material, whereby the electrostatic attractive force becomes weaker.
Also at the projection of the transfer conveying belt 54 formed by the supporting roller 58, the leading end of the transfer material is lifted by the rigidity thereof from the transfer conveying belt 54 and may be eventually peeled therefrom. In such a case, the transfer material may not be properly conveyed to the black image transfer portion to cause sheet jamming, or the leading position of the image on each transfer material may fluctuate because the way of the transfer material is varied by such sheet peeling whereby the image is transferred at a timing different from the originally designed timing.
An object of the present invention is to provide an image forming apparatus capable of preventing the peeling of the transfer material from the transfer material bearing member, or the aberration in the position of the transfer material borne on the transfer material bearing member.
Other objects of the present invention, and the features thereof, will become fully apparent from the following detailed description, which is to be taken in conjunction with the accompanying drawings.